Bluetooth® is an open standard for creating and exchanging data over ad hoc short-range wireless communication networks. Developed by Ericsson in 1994, Bluetooth has become a standard feature of many wireless communication terminals, as well as many other types of portable electronic devices. A design goal of all portable electronic devices is to reduce power consumption, thus prolonging battery life.
A digitally modulated transmitter power amplifier for a Bluetooth system uses polar modulation, with phase information modulated onto a high frequency periodic signal (clock) on the carrier frequency, and digital coded amplitude modulation. The number of bits used for the amplitude modulation is related to the Digital to Analog Converter (DAC) quantization noise allowed according to the specification.
The number of bits used for modulation relates to the number of circuits needed, all switched on a high frequency clock. High frequency switching consumes relatively large currents and hence increases power consumption. Reducing the number of modulation bits reduces the supply power consumption; however, this increases the quantization noise, and vice versa.
The carrier frequency noise specification for a Bluetooth output signal is an absolute specification. This means the noise specification is toughest on the maximum output power, which defines the minimum number of modulation bits needed in this case. The number of modulation bits can be reduced when lower than maximum output power is required. The lower number of modulation bits causes the Most Significant Bit (MSB) cells of the power amplifier to be disabled, because these MSB data bits have a value “zero.” However, the high frequency clock buffers for these unused MSB cells, when still active, do dissipate power. This wasted power increases power consumption, increases heat that must be dissipated, and decreases the useable battery life of portable devices.